Multiphase processes involving the reaction of gas and liquid phase reactants on a solid catalyst form the basis for production of a large variety of intermediate and end products, as examples, in the manufacture of monomers and pharmaceuticals and in crude oil processing. From the perspective of the gases used in such processes, a number of commercially important gas-liquid reaction systems can be considered for instance that involve the reaction of hydrogen with a liquid substrate (hydrogenation, hydrogenolysis, hydrotreating), that involve the reaction of oxygen or a similar oxygen-based gaseous reactant (e.g., ozone) with a liquid substrate (oxidation, ozonolysis), that involve the reaction of a halogen gas with a liquid substrate (fluorination, chlorination) or that involve the use of a combination of gases (hydroformylation).
Solid catalysts are desirably used in both gas-liquid and in liquid-liquid reaction systems for facilitating the separation and recovery of spent catalyst and the processing of crude reaction products, but gas-liquid reaction systems frequently pose difficulties in terms of getting a gaseous reactant into a liquid and to a heterogeneous solid catalyst surface. As a result, in certain types of multiphase processes particularly (as elaborated in greater detail hereafter), substoichiometric gas to liquid reactant ratios can occur in the presence of the catalyst, so that undesirable side reactions can be catalyzed of liquid phase components at the solid catalyst interface. As well, replenishment of the gas reactant(s) is difficult as the gas reactant(s) is spent in these undesirable side reactions. Further, catalyst lifetime can be shortened by the interaction of the liquid reactant(s) with the catalyst.
The relative overabundance of a liquid reactant as compared to a gaseous reactant (which can be an alternative way of considering a substoichiometric gas to liquid reactant ratio condition) can particularly be an issue with those continuous reaction systems that are characterized by limited axial mixing from inlet to outlet, i.e., that are plug flow or quasi-plug flow in nature, as intrinsically a liquid substrate concentration is high at the inlet and lower at the outlet of the reactor, as well as in semibatch reaction systems, since again intrinsically a liquid substrate concentration is greatest at the start of a batch and lower at the end.
These and other complexities and difficulties of carrying out multiphase processes involving a solid catalyst and gas and liquid phase reactants are well documented in the literature and well-known to those in the art. In Trickle Bed Reactors: Reactor Engineering & Applications, V. V. Ranade, R. V. Chaudhari and P. R. Gunjal, Elsevier, Amsterdam (2011), for example, the advantages and disadvantages of various multiphase reactor systems are discussed at pages 9-12, including slurry reactors (where the reaction is carried out between mobile catalyst particles, gas, and liquid phases) and fixed-bed reactors (where the reaction between gas and liquid phase reactants takes place on or at the stationary catalyst).
In contrast, well-mixed slurry reactors (whether operated continuously or, as is more common, in a batchwise manner) facilitate effective temperature control within the reactor and are characterized by intensive mass transfer between all phases. However, as discussed in the cited reference, such well-mixed slurry reactors pose difficulties in terms of the separation of the product from the catalyst due to catalyst attrition, in terms of abrasion of equipment surfaces by moving catalyst particles, in terms of low specific productivity per unit volume, and in terms of use in a continuous mode taking into account catalyst separation and regeneration needs.
Accordingly, fixed bed reactors have been viewed as favored especially for continuous processes, however, these reactors have their own limitations, complexities and disadvantages in the context of carrying out multiphase processes involving reactants in the gas and liquid phases and a solid catalyst.
The substoichiometric issues described above are among the limitations, complexities and disadvantages recognized and discussed in a general frame of reference in Datsevich et al., “Multiphase fixed-bed technologies: Comparative analysis of industrial processes (experience of development and industrial implementation)”, Applied Catalysis A: General, vol. 261, pp 143-161 (2004) (hereinafter “Datsevich”), in particular on page 148 and following in relation especially to FIG. 4, wherein Datsevitch observes that in the initial part of a multiphase fixed bed reactor wherein the gas and liquid phase reactants first combine there is a significant stoichiometric excess of the liquid reactant both in the bulk liquid as well as at the catalyst surface, so that the reaction in this initial part is limited by mass transport resistance of the gas into the liquid (especially for gases such as hydrogen and oxygen having “very bad solubility in liquids”). At the same time, in the latter part of the reactor, Datsevich observes that these systems are mass-transfer-limited by the liquid compound. Datsevich also observes that along the axial flow of a fixed bed multiphase gas-liquid reaction system there is a point at which there is an “ideal” correlation of the concentrations of the gas and liquid reactants, wherein the concentration of a liquid phase reactant corresponds to the stoichiometric concentration of a gaseous reactant on the catalyst surface, so that for active catalysts both concentrations are zero at the catalyst surface.
The complexities of carrying out these multiphase processes in a continuous fixed bed reactor are explored experimentally and through math modeling in a more specific context of interest for the present invention in a preferred application, in Kilpio et al., “Experimental and Modeling Study of Catalytic Hydrogenation of Glucose to Sorbitol in a Continuously Operating Packed-Bed Reactor”, Ind. Eng. Chem. Res. 2013, vol. 52, pp 7690-7703, wherein temperature- and concentration-dependent reaction kinetics, catalyst deactivation, internal diffusion and heat conduction within the solid catalyst particles, radial heat conduction and mass dispersion in a selected reactor section, liquid holdup, gas-liquid mass transfer, pressure drop and axial dispersion were evaluated in, and used to math-model, a lab scale (1.15 cm diameter, 7 cm long) continuous flow packed bed reactor containing 0.5 grams of a commercial ruthenium on carbon catalyst.
In recognition of the above-mentioned complexities and mass transfer related limitations of continuous fixed bed multiphase reaction systems, continual efforts have been made to improve the performance of such systems.
Datsevich proposes one refinement to address and reduce mass transfer limitations from gas to liquid, using instead a “saturator” before the reactor to accomplish the mass transfer, so that only a liquid phase saturated with gas is fed into the reactor. In effect, the reactor volume in the initial part of a fixed bed reactor holding the gas phase is avoided altogether. Datsevich observes that since the solubilities of gases such as hydrogen and oxygen are low and the concentration of the liquid reactant in the feed is comparatively higher than the equilibrium gas concentration in the liquid phase, recycling of the final product through the saturator is necessary to deliver the needed quantity of gas to the reaction zone. Datsevich thus effectively takes the approach of diluting the inlet or starting concentration of a substrate in the liquid phase (using the product or a portion thereof as the diluent), so limiting the liquid substrate's availability to the heterogeneous catalyst in a corresponding way as a gaseous reactant's availability to the catalyst is limited by gas solubility and resistance to mass transfer considerations.
A series of published applications and issued patents to Michael D. Ackerson and others, see, e.g., US 2012/0184789 to Ackerson et al.; U.S. Pat. No. 7,569,136; U.S. Pat. No. 7,291,257; U.S. Pat. No. 6,881,326; U.S. Pat. No. 6,428,686; and U.S. Pat. No. 6,123,835, are of a very similar nature, wherein various methods of hydroprocessing both petroleum and non-petroleum feedstocks (US 2012/0184789) are described in which a diluent is fed with hydrogen and a feedstock in need of hydroprocessing so that substantially all of the feed and hydrogen are in a single, continuous liquid phase as a hydrogen-gas-free liquid feed stream to the reactor. As in Datsevich, the diluent can be at least a portion of a cooled and/or separated reaction product that is recycled.
In effect, both Datsevitch's presaturated one-liquid-flow (or POLF) technology and Ackerson's process technology operate in the liquid mass transfer-limited region of FIG. 4 in Datsevitch, and use dilution and significant product recycling to cope with throughput decay while avoiding the substoichiometric issues in the initial part of the reactor in a conventional fixed bed multiphase reaction system that have been mentioned above. However, these types of approaches do intrinsically involve some loss of productivity in the use of dilution, as well as significant costs for the substantial recycle that is required especially for low-solubility gases such as hydrogen and oxygen.
Because of this recycle aspect of Datsevich's and Ackerson's approaches, these approaches have however been recognized as ill-suited for chemical processes involving high rates of gas consumption in that enormous product recycle rates (or equivalent dilution, diluent recovery and recycle for non-product diluents) would be required. Modifications of the POLF concept have accordingly been proposed wherein some gas would enter the reactor in the gas phase so that as gas in the liquid phase in consumed by reaction, a constant gas concentration in the liquid phase would be maintained through the whole of the fixed bed, see, e.g., DE 102006044579, RU 2083540 and WO 03091363.
Another approach to the particular substoichiometric issues described above would be to improve the solubility and/or availability of gas phase reactants in the liquid phase. Though not in regard to a process involving a solid catalyst, US 2013/0240781 A1 to Subramaniam et al., for example, reports a method for increasing the ozone concentration in a liquid, and then using the increased ozone concentration liquid for performing (in the absence of a catalyst) ozonolysis of a substrate. As related by Subramaniam et al., ozonolysis has typically been performed by bubbling ozone through an aqueous phase or through an organic liquid phase containing a substrate. However, these traditional methods are described as having certain drawbacks. Since ozone is highly reactive, the reaction temperature must be subambient (close to 0 degrees Celsius), but ozone is of limited solubility in a liquid phase at these temperatures. Further, ozone tends to react with many traditional organic solvents that might be used, resulting in waste products and further limiting ozone availability for conducting the reaction. The solution offered by Subramaniam et al. is to introduce ozone into an inert liquid under circumstances wherein the ozone/liquid combination has a temperature between about 0.8 and 1.5 times the critical temperature of ozone, and increasing isothermally the pressure of the ozone-containing gas above the liquid to about 0.3 to about 5 times the critical pressure of ozone so as to increase the solubility of the ozone in the liquid. The pressure is controlled to tune the solubility of the ozone in the liquid.
U.S. Pat. No. 7,365,234 by Subramaniam et al. adopts a similar approach in the context of the catalytic hydroformylation of olefinic feedstocks, wherein an olefin is reacted with CO and H2 in the presence of a hydroformylation catalyst in a liquid that has been volumetrically expanded with a compressed gas, typically supercritical or subcritical (near critical) carbon dioxide added generally to the limits of the solubility of a homogeneous Rh-based catalyst, to tunably increase the amount of CO and H2 available for reaction in the liquid phase. Surprisingly, altering the amount of the compressed gas in the liquid phase alters the chemoselectivity of the products, and varying the content of the compressed gas in the liquid allows higher ratios of the more desired linear aldehyde to less desired branched aldehyde products to be realized.
In hydroprocessing, too, processes are known wherein a hydrogen donor solvent or another material is used to improve hydrogen transfer and availability into the reacting liquid phase, however, at the cost of requiring additional separation and recovery/recycle steps, or in relation to the Subramaniam references with the requirement of operating under certain near critical ranges of conditions with the added costs associated with achieving and maintaining these conditions.